Generating thrust without fuel relies on missing details

A system called the EmDrive requires an exquisite balancing of forces.

A new system called the EmDrive, a way of using electricity to generate thrust without the need for fuel, is one of those ideas that will generate a lot of heat and noise, but probably not a lot of thrust for many years to come. I had never heard of the idea until today, and the latest paper, a translation, doesn't throw a lot of light on the physics itself. So brace yourself and let's see what we can figure out from a grainy line drawing in the translation.

The idea is based on a standing wave cavity: two mirrors—in this case, microwave mirrors—facing each other so that microwaves travel back and forth between them. In one picture, two waves are travelling in opposite directions between two mirrors. If you look at both waves simultaneously, however, you get another picture: a standing wave, like the vibration on a violin string. The key thing about this system is that these two pictures must be self-consistent. We will use that to examine the EmDrive's potential for thrust.

In our simple picture of two mirrors facing each other, the mirrors are subject to equal and opposite forces. Essentially, the photons reflect from each mirror, exerting a force in doing so. Because this is symmetric, the cavity cannot have a net force. Even if we were to make the light very focused at one end and very diffuse at the other, we are simply distributing the same number of photons differently at each end, so the total force remains the same.

The key claim to the EmDrive is that it breaks this symmetry, allowing a net force to exist.

We can consider the EmDrive as having three mirrors. One is large and facing the other two. The other two are arranged at 45 degrees, so light from the big mirror hits these mirrors at 45 degrees. In this arrangement, the force imparted by the photons is divided up. Only half the force generated at the small mirrors is directed opposite to the large mirror, so it doesn't balance the force exerted by the photons hitting the large mirror. As a result, it seems that we should have thrust. In the standing wave picture, then, things look good.

Except that the photons reflected by the small mirrors don't return directly to the main mirror—they need to bounce off the other angled mirror before they can do so. So each photon applies half the force twice, which should balance that from the main mirror. This simple picture is why, on first glance, it appears that this will never generate a net force.

Put in a more general context, my feeling is that any stable optical resonator cannot exert a net force because it requires certain symmetries. But an unstable resonator certainly can. This is important, because the second part of the claim—though it's not made in the article—is that one can start to generate thrust by making the resonator better and better. Essentially, the longer the photon circulates in the cavity, the more net force it imparts.

You can think of it like this: if a resonator allows a photon to circulate a thousand times before it escapes and you attach a one Watt microwave source to the resonator, for each Watt that leaves the resonator, one thousand Watts are stored in it. It would seem that if we increase the quality of the resonator enough, every photon will contribute multiple times to generating the force, and we would be on to a winner.

But there's a problem here, in that some combination of two things must be happening to destabilize the resonator: each time a photon imparts a force to a mirror, the frequency of the light has to drop (the reflected light is ever so slightly red-shifted compared to the incoming light); alternately, the photon is absorbed by a mirror. If the quality of the resonator is high, then as soon as the photon shifts in frequency even a bit, it will get thrown out of the resonator. And if the photons are absorbed by the mirror, the quality is necessarily low. In either case, if the resonator is unstable, the photons don't stay in there for very long.

In the end, there is a limit to how much force this can impart. Each photon carries only so much energy, which can only exert so much force. Since a resonator is an energy storage device, when you use that energy to exert a force, you reduce the storage capability of the resonator. You cannot both exert a lot of force and store a lot of energy. Yet this is exactly what appears to be required.

What about the measurements in the paper? The measurements appear to back the claim up. Unfortunately, the work involves throwing around up to 2.5kW of power to measure less than a Newton of net force. In those conditions, little things like anisotropic thermal expansion become absolutely critical to the measurement. Quite simply, there isn't enough detail in the experimental setup and not enough tests against possible problems to trust the data yet.

There is also a further missing point: the quality of the resonator is measurable, and the mode—think of modes as the path the radiation takes inside the resonator—can be calculated. The measurements in the paper could and should have been compared to a calculation.

My opinion is that this is unlikely to ever work as intended, especially not in a challenging environment like space. I believe that if the microwave resonator is unstable, a net force can be produced, but in a stable cavity no net force can exist. Certainly, the idea that a high quality resonator can impart more force than a low quality resonator cannot be true—at least not if we also want to conserve energy.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku

How much force would you get from a directed microwave beam generator of 2.5 KW versus what this thing might generate.

Not much - consider that current ion drives usually put out as much force as the weight of piece of paper lying on your hand @ Earth gravity.

This would probably put out significantly less, if it worked.

Quote:

Boeing's Phantom Works, which works on various classified projects and has been involved in space research, went as far as acquiring and testing the EmDrive, but say they are no longer working with Shawyer.

I'm really confused as to how this is supposed to work, and be better than a laser pointing backwards? Getting net thrust out of a totally closed system seems logically impossible to me in our current understanding of physics. If that is what the claim is here, I'm going to lump this in with perpetual motion machines.

I'm really confused as to how this is supposed to work, and be better than a laser pointing backwards? Getting net thrust out of a totally closed system seems logically impossible to me in our current understanding of physics. If that is what the claim is here, I'm going to lump this in with perpetual motion machines.

This technology was first developed by the KGB in the late 1950s. JFK sent agents to Moscow to steal the data but they were caught and gave up the plan - and look what happened to him a few years later. Some data was then sold in the 1970s to a shell company of Northrop-Grumman backed with venture capital from Elvis Presley. They were found out too, and look what happened to Elvis. Coincidence? You decide.

The solar-sail-industrial-complex is trying to keep this technology out of the public eye. Beware, Ars editors! They will be coming for you next!

The trick being to exert more force than just letting the photos go out the back, as in just aiming a laser behind you.

This "trick" is, sadly, unphysical.

An electric reactionless drive would be awesome, because you don't need fuel to generate electricity, solar panels will do it. The big problem is that reactionless drives are as unphysical as perpetual motion machines because Einstein says so.

Remember that e=mc^2 bit? Yeah, the whole "speed of goddamned light squared" on the "mass" side of things? Means you need an enormous amount of energy (as photons) to do the work that a small amount of mass can do in terms of reaction impulse.

It sounds like firing a laser would be a more effective means of generating thrust... the problem being that you still need some kind of fuel to actually power the laser.

Well, unless this depends on ambient photons, it needs a power source the same as the laser. I think in this case, "fuel" means "mass ejected from the back end", not necessarily that nothing is consumed to produce thrust.

The choice of word "fuel" is misleading. Fuel suggests energy, but the claimed innovation of this paper is more subtle. Half the other posts here are talking about conservation of energy being violated (search "perpetual motion", "getting more energy") or agreeing with such posts. You should rename the story:"Generating thrust without propellant relies on missing details"

This seems like pointing a fan at a sailboat and expecting the sailboat to move... except that the fan is mounted on the sailboat. You might think you can cleverly 'destabilize' the fan/sail system to produce a net force, but it just never works out that way.

This device simply cannot work! No matter what angle, shape, texture, coatings, power is used in its construction, there's simply no system defined within our current understanding of physics would allow this to work without expelling something or pushing against the Earth's magnetic field (which it doesn't purport to do).

But if it does expel matter or energy or even rely on pushing off against another force, then it's just another variety of reaction engine just like a chemical rocket or electrodynamic tether. Only much more inefficient.

And this "anti-gravity" stuff is all very shady to begin with. If gravity were really a "force" in the conventional sense of the term, then there might conceivably be an anti-"force" for it. But it's not.

Gravity is a function of the geometry of spacetime itself so nothing short of altering spacetime in someway would allow you to propel yourself without violating Newton's third law. I.E. Creating some kind of a spacetime bubble like the Alcubierre (warp) Drive.

I'm sorry, but this is bullshit. Light does carry momentum, at microwave energies the Poynting vector describes this best. So it is perfectly fine to shine light out the back for a rocket, not very effective, but it works. But here no light escapes, so if you draw a box around the system noting is leaving, there is nothing to carry the momentum (except for an asymmetric thermal glow the box, but that is a *tiny* amount of force).

But it gets worse. Conservation of momentum is equivalent to saying the laws of physics are the same in different places. For their system to work as described the laws of physics would have to be different at one end of the cavity than the other. By a large amount. No, there is no way to get around this conclusion.

This seems like pointing a fan at a sailboat and expecting the sailboat to move... except that the fan is mounted on the sailboat. You might think you can cleverly 'destabilize' the fan/sail system to produce a net force, but it just never works out that way.

Drives like this isn't how we'd get there. Really nuclear pulse propulsion is the best means of interstellar travel we've got; we could do it today if we really wanted to. Or rather, do it in twenty years. Or ten if we really cared.

Forget about force and energy, let's stick with conservation of momentum. If we start out with the engine stopped, the total momentum of the engine at a given time (and whatever else is attached to it) will equal the total momentum of the photon stream that has been ejected out of the back. (This assumes the engine is used in space, where air resistance or friction are an issue, some momentum will be transferred out of the engine to the air/earth.) Internal reflections are meaningless, until a photon leaves the system, it doesn't matter.

If you have the rate of photon ejection dn/dt and the momentum per photon p, you can calculate the total change in momentum of the ejected photon stream per unit time, p dn/dt. This is equal to the total force exerted by the engine on the photons, and by Newton's Third Law, it is also equal to the force exerted on the engine.

The basic idea of an engine that emits photons to move has been considered before. I believe you need a nuclear reaction to generation any meaningful thrust (the total thrust is very small, but in space this could be useful for something like station keeping at an unstable Lagrangian point).

The only nitpick I'm qualified to diagnose - its generating thrust without *reaction mass*, not fuel. Fuel is used to power the reactor that generates the electricity, reaction mass is the stuff that gets thrown out the back to make the rockect go forward.

I'm going to have to disagree with the analysis in the article. Thrust really isn't going to have anything to do with stability of the resonator, or even whether or not the chamber is a resonator. This is simply conservation of momentum.

Imagine the same thing with ping pong balls inside. You're not going anywhere unless one of the balls somehow shoots out the back. Even if you ramp the balls up to really high speeds. In this case, the physics doesn't change just because it's photons instead of ping pong balls.

The resonance within the chamber is simply a red herring, hiding the fact that this thing simply doesn't (and couldn't) work the way its described.

Isn't this more one of those "you build it and it works but you don't understand it so you pretend it doesn't work on paper" type things?

For all the people calling quackery on this for the last 5 or 6 years you'd think refuting it would be easy.

Instead some random engineers and Chinese scientists make something that to any reasonable observer seems to limp along while researchers explain repeatedly that it can't limp...

Meanwhile those quoted always jump to the 'conservation of momentum is violated! blasphemy!' page instead of building one of these stupid things and then figuring out how the experiment is bad or if there's some effect that could explain it.

It really feels like the skeptics are more interested in armchair quarterbacking than debunking.

And it's not like I know what to think or have any relevant knowledge.

But you'd think after 6 years if it was going to be another neutrino faster than light debacle it'd have been exposed with evidence instead of insinuation by people who haven't even tried building the system or evaluating it.

I'll bet anything there's eventually an explanation that lets these stupid things spin on their test beds without violating any crazy laws of physics.

I'll bet anything there's eventually an explanation that lets these stupid things spin on their test beds without violating any crazy laws of physics.

Having already stated that you have no relevant knowledge, why would you possibly think this? Because you want it to be true?

No one has bothered building one because the experimental flaws are incredibly obvious just from reading the papers. Not to mention that these are very fundamental aspects of physics being violated. They aren't tweaking quantum mechanics in some new way; they're trying to mess with things that are well established. You need better proof than "I ran an experiment with poor methodology and got a tiny measurement" to overturn that.

If this worked, then a rarefied ideal gas in a trapezoidal box should also generate thrust. The simple question to ask in these situations is: are the physics translationally invariant? Clearly where this box is has no affect on its operation. Translational invariance implies a conserved quantity: inertia. So if this box conserves inertia, any thrust would need to be balanced out (thrust is a current of inertia). Therefore, basic physics predicts that such a device will NEVER work. Either basic physics is wrong, or some assumption is wrong.

See if you can spot the problem with this inertial thruster: A laser is aimed at a solar cell. The two are mechanically and electrically attached. While the photons from the laser are in flight to the solar cell, their momentum is equal to the momentum of the laser and solar cell apparatus. Therefore, the speed of the apparatus should be proportional to the number of photons in flight between the laser and solar cell. Why wouldn't this work?

This device simply cannot work! No matter what angle, shape, texture, coatings, power is used in its construction, there's simply no system defined within our current understanding of physics would allow this to work without expelling something or pushing against the Earth's magnetic field (which it doesn't purport to do).

But if it does expel matter or energy or even rely on pushing off against another force, then it's just another variety of reaction engine just like a chemical rocket or electrodynamic tether. Only much more inefficient.

And this "anti-gravity" stuff is all very shady to begin with. If gravity were really a "force" in the conventional sense of the term, then there might conceivably be an anti-"force" for it. But it's not.

Gravity is a function of the geometry of spacetime itself so nothing short of altering spacetime in someway would allow you to propel yourself without violating Newton's third law. I.E. Creating some kind of a spacetime bubble like the Alcubierre (warp) Drive.

Sorry but the Casimir effect proves that energy can be converted directly into momentum. Not that this device does anything like that.

Meanwhile those quoted always jump to the 'conservation of momentum is violated! blasphemy!' page instead of building one of these stupid things and then figuring out how the experiment is bad or if there's some effect that could explain it.

It really feels like the skeptics are more interested in armchair quarterbacking than debunking.

You need better proof than "I ran an experiment with poor methodology and got a tiny measurement" to overturn that.

Especially when the list of -possible- (but unexplored) influences is obvious and long. Were accurate temperature measurements taken of each mirror? At a significant number of locations? How does it -scale-? Both physically and power-input wise?

Meanwhile those quoted always jump to the 'conservation of momentum is violated! blasphemy!' page instead of building one of these stupid things and then figuring out how the experiment is bad or if there's some effect that could explain it.

It really feels like the skeptics are more interested in armchair quarterbacking than debunking.

Isn't this more one of those "you build it and it works but you don't understand it so you pretend it doesn't work on paper" type things?

For all the people calling quackery on this for the last 5 or 6 years you'd think refuting it would be easy.

Instead some random engineers and Chinese scientists make something that to any reasonable observer seems to limp along while researchers explain repeatedly that it can't limp...

Meanwhile those quoted always jump to the 'conservation of momentum is violated! blasphemy!' page instead of building one of these stupid things and then figuring out how the experiment is bad or if there's some effect that could explain it.

It really feels like the skeptics are more interested in armchair quarterbacking than debunking.

And it's not like I know what to think or have any relevant knowledge.

But you'd think after 6 years if it was going to be another neutrino faster than light debacle it'd have been exposed with evidence instead of insinuation by people who haven't even tried building the system or evaluating it.

I'll bet anything there's eventually an explanation that lets these stupid things spin on their test beds without violating any crazy laws of physics.

Yeah! Don't count this out just because other people weren't able to replicate the results! I'm sure their methodology was sound.

Why is this on Ars, doesn't Ars have some sort of science standards or common sense? It requires very little education in physics to see past the hand waving to the basic fact that without new laws of physics, this cannot work. Are they proposing new laws of physics? No. Are they claiming their device violates conservation of momentum? Of course!

Why is this on Ars, doesn't Ars have some sort of science standards or common sense? It requires very little education in physics to see past the hand waving to the basic fact that without new laws of physics, this cannot work. Are they proposing new laws of physics? No. Are they claiming their device violates conservation of momentum? Of course!

Did you read the article? Those very flaws are exactly why this is on Ars. Journalism isn't just reporting accurate findings, it's also about quashing the poor ones.